Would you mind let us know what is your current infrastructure network devices so that we can give right solution to you, whether you can use VPN or using RDP Windows remote desktop or any other in you existing network.

Do not undermine the Windows Remote Desktop, if it is correctly configured, you can access files securely from remote network.

However it has to go step by step to give the perfect solution in your case, once you let us know your existing network devices.

Laptops which run Windows XP professional need to access files on the office network remotely. This access needs to be secure. Would the office need static IP addressing for this to work? Is there a better system than static IP addressing?